To evaluate glufosinate resistance, we conducted a dose-response bioassay using two glyphosate-resistant Italian ryegrass (Lolium multiflorum Lam.) populations (S63 and S64) in Shizuoka, Japan. Two additional cultivars, Waseyutaka and Common, were used as the susceptible control in the experiment. At the five to six leaf stage, glufosinate was applied at a rate of 0, 0.25, 0.5, 1.0, or 2.0 kg active ingredient (a.i.) ha–1 for each of the susceptible cultivars, at a rate of 0, 0.5, 1.0, 2.0, or 4.0 kg a.i. ha–1 for S63, and at a rate of 0, 1.0, 2.0, or 4.0 kg a.i. ha–1 for S64. Almost all individuals of the two susceptible cultivars died after the application of 1.0 kg a.i. ha–1,
the recommended rate for pest control, but 33 and 23% of individuals survived from the S63 and S64 populations, respectively. The ratios of the LD50 values for resistant vs. susceptible populations (R/S ratios) for S63 and S64 were 1.3 and 1.8, respectively. However, because Italian ryegrass is an obligate out crossing, wind-pollinated species, the S63 and S64 populations included both susceptible and resistant individuals and thus the R/S ratios may have been underestimated.